The present invention relates in general to cutting tools for metal cutting and particularly to indexable inserts (throwaway tips) used for finish cutting by turning.
FIG. 10 shows a positional relation between a cutting tool (hereinafter will be also referred to as indexable insert or simply as insert) 1 used for finish cutting or machining of a material having a large hardness and a work 100 to be finished thereby when observed from the tool face 7 side. By the hatched part is indicated the cross sectional shape of a chip. The indexable insert 1 has at a nose 2 thereof a circular or curved cutting edge 3 when observed in plan. The indexable insert 1 used in this kind of finish cutting is generally used at a relatively small depth h of cut which is equal to or smaller than xc2xd of the nose radius (i.e., the radius of the curved cutting edge 3 at the nose 2). Accordingly, the chip usually becomes thinnest at a portion which is cut by a cutting edge portion adjacent a front relief terminal portion M and thickest at a portion which is cut by a cutting edge portion adjacent a side relief terminal portion Y.
In such cutting, the cutting force applied to the cutting edge of the cutting tool is inversely proportional to the thickness of the chip. Since the cutting tool is subjected to a large cutting force at or adjacent the side relief terminal portion Y, chipping or breakage of the cutting edge is liable to occur at or adjacent the side relief terminal portion Y. Thus, the cutting tool used for this kind of cutting is, in many cases, processed by a cutting edge treatment such as chamfering and honing. It is usual practice that such chamfering has a uniform width when observed along the cutting edge from the tool face side. The width of the chamfer is herein used to indicate the width (size) of the chamfer when observed in plan from the tool face side.
In case finish cutting is done with such a prior art cutting tool, i.e., a tool having a uniform chamfer along the cutting edge, there occurs the following problems. Namely, finish cutting is usually done only at the nose, i.e., by the curved cutting edge at the nose. Such a cutting tool does not have at the side relief terminal portion a chamfer of the width corresponding to the thickness of the chip, thus causing a problem that chipping or breakage of the cutting tool is liable to occur. Although finish cutting is done by the curved cutting edge at the nose, the depth of cut varies at all times. Thus, if the width of the chamfer is constant, an increase in the depth of cut at or adjacent the side relief terminal portion causes a relative decrease in the width of chamfer thereat, thus causing a decrease in the strength. On the other hand, if the depth h of cut is decreased too much, the width of the chamfer at a cutting edge portion adjacent the front relief terminal portion becomes too large, thus deteriorating the roughness of the finished surface.
Under these circumstances, there has been proposed a cutting tool having at the cutting edge a chamfer that increases as it goes away from the nose as disclosed in Japanese Utility Model Publication No. 53-122790. However, the chamfer of this cutting tool is uniform in width at the nose (i.e., at the curved cutting edge) and adapted to increase in width gradually at the sides of the cutting tool (i.e., at the straight cutting edges). The cutting tool is intended for rough cutting with the depth of cut larger than the nose radius. Further, there has been proposed a cutting tool that has a chamfer at the entirety of the nose portion. However, the chamfer is formed into a nearly semi-circular shape when observed from the tool face side. Thus, the width of chamfer of the cutting edge along the nose varies so largely, thus causing a problem that the roughness of the finished surface is deteriorated.
It is accordingly an object of the present invention to provide a cutting tool used with a depth of cut that causes only a curved cutting edge or a portion thereof to be used for cutting, which can attain an elongated life without incurring a decrease in the surface roughness.
To accomplish the above object, there is provided according to a first aspect of the present invention a cutting tool comprising a curved cutting edge at a nose, a pair of straight cutting edges between which the nose is located, and a chamfer extending along the curved cutting edge and the straight cutting edges, wherein a portion of the chamfer extending along the curved cutting edge forms a curved ride at a junction with a tool face and increases in width from a place located at or adjacent a junction of the curved cutting edge and each of the straight cutting edges to a placed located at or adjacent a midpoint of the curved cutting edge when observed in plan.
With the cutting tool structured as above, it becomes possible to obtain such a width of chamfer that can be varied in proportion to the thickness of chip that in turn varies depending upon a variation of the depth of cut and the feed, thus making it possible to obtain an optimal width of chamfer at each place of cut. Namely, in finish cutting by turning, the depth of cut is selected so that the cutting is performed by a portion of the cutting tool that is located within the curved cutting edge at the nose. By the cutting tool structured as above, it becomes possible to perform cutting by positioning a circular or curved cutting edge portion smaller in the width of the chamfer on the front relief terminal side and a curved cutting edge portion larger in the width of the chamfer on the side relief terminal side, thus making it possible to elongate the life of the cutting tool without incurring a decrease in the surface roughness. In the meantime, while the width of the chamfer decreases gradually from the midpoint of the curved cutting edge to the junction of the curved cutting edge and each of the straight cutting edges, a decrease in the strength of the cutting edge is not actually caused since the depth of cut at finish cutting is mostly xc2xd of the nose radius.
According to a second aspect of the present invention, there is provided a cutting tool comprising a curved cutting edge at a nose, a pair of straight cutting edges between which the nose is located, and a chamfer extending along the curved cutting edge, wherein the chamfer increases in width from a place located at or adjacent a junction of the curved cutting edge and one of the straight cutting edges to the other of the straight cutting edges when observed in plan.
As described above, the width of the chamfer of the cutting tool according to the first aspect of the present invention increases toward the middle point of the curved cutting edge so that the difference in the direction of feed does not cause any difference in the effect of the cutting tool. In contrast to this, the cutting tool according to the second aspect of the present invention produces different effects depending upon the direction of feed. However, since the chamfer of the cutting tool according to the second aspect of the present invention can have an increasing width in accordance with an increase of the thickness of the chip since the width of the chamfer of the cutting tool is adapted to increase gradually from one straight cutting edge to the other straight cutting edge, thus making it possible to obtain an optimal width of chamfer at each place of cut.
According to a third aspect of the present invention, there is provided a cutting tool comprising a curved cutting edge at a nose, a pair of straight cutting edges between which the nose is located, a pair of wiper edges disposed between the curved cutting edge and the respective straight cutting edges, and a chamfer extending along the curved cutting edge and the wiper edges, wherein a portion of the chamfer extending along the wiper edges is smaller in width than a portion of the chamfer extending along the curved cutting edge when observed in plan, and wherein the portion of the chamfer extending along the curved cutting edge increases in width from a junction of the curved cutting edge and each of the wiper edges to a midpoint of the curved cutting edge when observed in plan.
With the cutting tool having the wiper edges between the curved cutting edge and the respective straight cutting edges, the roughness (surface quality) of the finished surface can be improved when finish cutting is carried out by using the wiper edge. On the other hand, since a large thrust force results from the cutting by using the wiper edge, it is necessary to make basically smaller the width of the chamfer. However, when the width is made smaller throughout the entirety of the chamfer, the width of the chamfer becomes insufficient at the side relief terminal portion so that breakage of the cutting tool is liable to occur. In this connection, with a varying with of chamfer, the surface roughness can be improved without lowering the strength.
According to a fourth aspect of the present invention, there is provided a cutting tool comprising a curved cutting edge at a nose, a pair of straight cutting edges between which the nose is located, a wiper edge disposed between the curved cutting edge and one of the straight cutting edges, and a chamfer extending along the curved cutting edge and the wiper edge, wherein a portion of the chamfer extending along the wiper edge is smaller in width than a portion of the chamfer extending along the curved cutting edge when observed in plan, and wherein the portion of the chamfer extending along the curved cutting edge increases in width from a place located at or adjacent a junction of the curved cutting edge and the wiper edge to the other of the straight cutting edges when observed in plan.
The cutting tool according to the third aspect of the present invention can produce the same effect irrespective of the direction of feed. In contrast to this, the cutting tool according to the fourth aspect of the present invention produces different effects depending upon the direction of feed. However, the cutting tool according to the fourth aspect of the present invention makes it possible to obtain an optimal width of chamfer at each place of cut.
In the meantime, the wiper edge is preferably rounded outward and in the form of a circular arc of the radius larger than the nose radius when observed in plan or in the form of a straight line. Further, when the wiper edge is in the form of a straight line, it is preferably arranged in parallel to the surface to be finished by turning (i.e., in case of cylindrical cut, in parallel with a generatrix of the surface to be cut). When the wiper edge is in the form of a circular arc, the radius of the circular arc is preferably 2 mm or larger. Further, when the wiper edge is in the form of a straight line and the cutting tool is moved sideways along the work, it is preferably arranged nearly in parallel with the direction of feed or arranged so as to have a back taper such that the wiper edge goes away from a line parallel to the direction of feed as it goes away from the nose. The angle of back taper is preferably 3 degrees or smaller.
In the meantime, the minimum of the width of the chamfer is preferably 0.2 mm or smaller and the maximum is preferably within the range from 0.05 to 0.5 mm.
Further, it is preferable that the width of the chamfer is determined so as to satisfy W2/W1xe2x89xa71.5 where W1 is the minimum width of chamfer and W2 is the maxi width of the chamfer.
Further, the angle xcex81 of the chamfer with respect to the tool face and at the place where the width of the chamfer is minimum is preferably within the range from 5 to 30 degrees, and the angle xcex82 of the chamfer with respect to the tool face and at the place where the width of the chamfer is maximum is preferably within the range from 15 to 45 degrees. Particularly, it is preferable that xcex81xe2x89xa6xcex82 is satisfied. In this connection, the angle of chamfer with respect to the tool face is intended to indicate the angle between the chamfer and the tool face when the tool face is flat.